Beryllium surface treatment

ABSTRACT

BERYLLIUM SURFACES ARE TREATED IN A TWO STEP PROCESS TO REMOVE CORROSION PRODUCTS AND PREVENT FURTHER CORROSION. THE PROCESS COMPRISES A CORROSION REMOVAL TREATMENT WHICH UTILIZES AN OXALIC ACID SOLUTION CONTAINING A SURFACTANT SUCH AS A LINER ALKYL ARYL SULFONIC ACID AND A PASSIVATION TREATMENT WHICH UTILIZES A SOLUTION CONTAINING PHOSPHORIC ACID, HEXAVALENT CHROMIUM IONS AND A SURFACTANT SUCH AS A LINEAR ALKYL ARYL SULFONIC ACID. THE PRESENCE OF THE SURFACTANT IN BOTH SOLUTIONS REDUCES THE SURFACE TENSION SO THAT THE SOLUTIONS PENETRATE THE POROUS STRUCTURE OF THE BERYLLIUM SURFACE RESULTING IN EFFECTIVE CORROSION REMOVAL AND PREVENTION OF FUTURE CORROSION.

3,827,919 BERYLLIUM SURFACE TREATMENT Robert F. Keafing, Syosset, N.Y., assignor to Sun Chemical Corporation, New York, N.Y. No Drawing. Filed Nov. 29, 1971, Ser. No. 203,025 Int. Cl. C23f 11/00 US. Cl 148-6.16 14 Claims ABSTRACT OF THE DISCLOSURE Beryllium surfaces are treated in a two step process to remove corrosion products and prevent future corrosion. The process comprises a corrosion removal treatment which utilizes an oxalic acid solution containing a surfactant such as a linear alkyl aryl sulfonic acid and a passivation treatment which utilizes a solution containing phosphoric acid, hexavalent chromium ions and a surfactant such as a linear alkyl aryl sulfonic acid. The presence of the surfactant in both solutions reduces the surface tension so that the solutions penetrate the porous structure of the beryllium surface resulting in effective corrosion removal and prevention of future corrosion.

BACKGROUND OF THE INVENTION This invention relates to the treatment of beryllium surfaces. In one of its more particular aspects this invention relates to the removal of corrosion products from beryllium surfaces and the prevention of further corrosion of said surfaces.

US. Pat. 3,301,718 to Simon J. Morana describes the passivation of beryllium, particularly powdered beryllium metal, by treatment with an aqueous mixture containing hexavalent chromium ions and phosphoric acid.

Beryllium, a strong lightweight metal has a pronounced tendency to corrode. Surfaces of parts and equipment fabricated from beryllium, particularly electro-discharge machined surfaces, also known as Eloxed surfaces are susceptible to corrosion and oxidation during exposure to moist atmospheres particularly salt fog or halogen-containing industrial atmospheres.

Previously electrolytic and electroless plating methods have been suggested for use in corrosion prevention, but such methods result in dimensional increases which are often disadvantageous. In addition such methods require costly fixturization due to the need for conforming anodes in order to insure uniform plating thickness especially on irregularly shaped parts.

Coating with organic films has only been partly successful because of the need to maintain critical dimensions which was rendered difiicult by reliance upon film coatings.

The use of a chemically applied chromium-containing coating became the preferred technique to protect the porous metal surfaces produced by electro-discharge machining. However, it was found that the commonly used chromium coating procedures had less effect than desired where highly corrosive atmospheres were encountered.

It is generally understood that one of the primary corrosion products of beryllium metal is beryllium hydroxide which is formed by the hydrolysis of beryllium as shown in the following equation:

Beryllium hydroxide appears as irregular spots of a snow-white material on the affected metal surfaces. Other forms of corrosion are also observed with beryllium surfaces. For example, at temperatures in excess of about 750 F. beryllium oxide (BeO) is formed.

It is accordingly an object of this invention to provide a process for removing corrosion products from beryllium metal surfaces and to prevent such surfaces from becoming corroded in the future.

nited States Patent O Another object of this invention is to provide such a process which is particularly effective for use with electro-discharge machined surfaces and other porous beryllium surfaces.

A further object of this invention is to provide a beryllium surface which is free of corrosion products and which will resist future corrosion.

Other objects and advantages of this invention will become apparent in the course of the following detailed disclosure and description.

STATEMENT OF THE INVENTION It has been found that beryllium surfaces, including especially porous beryllium surfaces which are produced as a result of the electro-discharge machining or Elox process can be protected from corrosion by means of a process which comprises a chemical corrosion removal treatment to remove corrosion products from the surface and a passivation treatment which is effective to protect the surface from future corrosion.

The corrosion removal treatment utilizes a dilute solution of oxalic acid containing a surfactant such as a linear alkyl aryl sulfonic acid in an amount sufficient to lower the surface tension of the oxalic acid solution and to thereby enable it to penetrate the pores of the beryllium metal surface, thus effectively removing even minute traces of corrosion products from pits and crevasses in the metal surface.

The passivation treatment utilizes in major proportions a solution of a hexavalent chromium ion-containing material and phosphoric acid and also contains a minor proportion of a surfactant such as a linear alkyl aryl sulfonic acid in an amount sufficient to lower the surface tension of the chromium-phosphoric acid solution thereby enabling the depositing of a chromium-containing coating upon the entire surface of the beryllium metal including the pits and crevasses thereof.

This overall treatment of the beryllium surface removes previous corrosion products and results in a thin adherent chromium-containing film which resists subsequent atmospheric corrosion for extended periods of time. A satin finish is provided on the beryllium metal surface which is highly resistant to corrosion due to corrosive atmospheres containing moisture and other corrosion producing materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Removal of corrosion from the affected surface is accomplished by applying to the surface the corrosion removal solution of this invention. Oxalic acid may be present in the solution in a concentration by weight of about from 5% to 25%. The surfactant may be in a proportion of about from 0.01% to 1% by volume of the oxalic acid solution. More than 1% may be used, but in general the use of greater proportions of surfactant results in increased foaming. A minimum amount sufficient to decrease the surface tension of the corrosion removal solution is necessary.

The alkyl aryl sulfonic acid is a biodegradable surfactant which improves the cleansing action and increases the chemical activity at the metal surface and in the pores of the metal. Suitable alkyl aryl sulfonic acids are straight chain alkyl benzene sulfonic acids in which the alkyl chain may be normal or iso and is C or longer. A typical example of such compound is dodecyl benzene sulfonic acid. These compounds are also generically known as linear alkylate sulfonates.

Other surfactants which are compatible with oxalic acid and which do not themselves adversely affect the beryllium surface can be similarly used.

Preyious to the corrosion removal treatment the beryllium surface is cleaned and dried. One convenient method for cleaning the surface is by means of an ultrasonic treatment using a fluorinated solvent. Such treatment should be continued as long as is necessary to insure that the surface is thoroughly cleaned. Other methods of cleaning may also be used. Immersion in any suitable solvent will generally remove the oil or other soil that may be present.

Drying is most conveniently accomplished by placing the parts to be treated in an oven. Treatment by baking at 160 F. for a period of about minutes is generally sufficient.

The corrosion removal step may be accomplished by applying the corrosion removal solution to the surface to be treated or by immersing the parts to be treated in the corrosion removal solution. The temperature of the solution may be varied from about 100 F. to 160 F. A preferred temperature is one in the range of 125 F. to 135 F. at which optimum corrosion removal action is achieved with minimum metal removal. In general less than 0.001 inch is removed at these optimum treatment temperatures.

The treatment with the corrosion removal solution may be continued for a period of about from 2 to 4 hours depending upon the extent of corrosion of the parts to be treated.

Following the corrosion removal step the corrosion removal solution is removed, most conveniently by rinsing the parts which have been treated with the solution with distilled or demineralized water. If desired the parts can also be ultrasonically cleaned for a relatively short period of time such as one or two 5 minute cycles in order to insure that the corrosion removal solution is completely removed.

The surface which has been stripped of the corrosion removal treatment solution is then immersed or otherwise treated with the passivation solution. This solution comprises a mixture of phosphoric acid, a solution containing hexavalent chromium ions and a surfactant which is compatible with such phosphoric acid and hexavalent chromium ion solutions and which does not itself adversely affect the beryllium metal surface being treated. A preferred class of surfactants are the above described alkyl aryl sulfonic acids.

In general equal volumes of concentrated phosphoric acid, such as 85% phosphoric acid and a saturated aqueous solution of potassium dichromate are used. The surfactant is added to the mixture of these two solutions in a proportion of about from 0.01% to about 1% by volume. About 0.1% by volume is preferred. The resulting mixture may then be diluted with up to about 2 parts distilled, demineralized or deionized water, per part of the resulting mixture if desired.

The temperature of the passivating solution can be varied within the range of about from 110 F. to 170 F. with temperatures in the range of about from 135 F. to 145 F. being preferred.

The passivation treatment is continued for a period of about 30 minutes to 1 hour in order to insure optimum deposition of the chromium-containing complex which protects the beryllium metal surface from future corrosion.

Following the passivation treatment the surface is rinsed using distilled or demineralized water in order to remove excess passivation solution. Optionally, cleaning with ultrasonics for a short period of time such as for one or two minute cycles can be employed.

Finally the parts which have been passivated and rinsed of the excess passivation solution are dried thoroughly to remove excess moisture. A convenient method of drying is by baking at atmospheric pressure or in a vacuum. Relatively complex parts can be dried by vacuum baking for a period of about 1 to 2 hours at 160 F. at 1 mm. pressure.

In some instances it may be desired to treat certain portions of the surfaces of metal parts while masking other areas. This can easily be accomplished by using conventional acid-resistant masking techniques such as plastic tape and strip-01f materials which are temperature and acid resistant on those surfaces which are not to receive the corrosion removal and passivation treatments.

The invention will be better understood by reference to the following illustrative examples.

EXAMPLE 1 Fifteen (15) beryllium test specimens containing areas 1.2 inches in diameter and from to inch deep which had been subjected to electro-discharge machining were divided into five groups of three specimens each as follows.

The Group 1 specimens 1, 2 and 3 were mildly etched in an oxalic acid solution and then passivated in a bath made by mixing equal volumes of a saturated aqueous solution of potassium dichromate and phosphoric acid solution which had been diluted with 2 parts water to 1 part acid.

The Group 2 specimens 4, 5 and 6 were treated according to the process of this invention by immersion in a 10% oxalic acid solution containing 0.1% by volume of a linear alkylate sulfonic acid (Nacconol 98 SA) for 2 hours at F. Specimens 4 and 5 were then passivated in a bath made by mixing equal volumes of a saturated aqueous solution of potassium dichromate and 85 phosphoric acid solution diluted with two parts dionized water to one part acid, and 0.1% by volume of a linear alkylate sulfonic acid (Nacconol 98 SA) at a temperature of 200 F. for 30 minutes. Specimen 6 was passivated in the same bath at room temperature for 30 minutes.

The Group 3 specimens 7, 8 and 9 were treated with the same solution as the Group 2 specimens. The passivation was continued for 30 minutes at F.

The Group 4 specimens 10, 11 and 12 were treated the same as the Group 3 specimens and then were painted with a polyester enamel using black velvet paint with zinc chromate plus wash primer. The parts were then subjected to a thermal vacuum exposure.

The Group 5 specimens 13, 14 and 15 were treated in the same manner as the specimens of Group 4 except that they were not exposed to thermal vacuum.

The test specimens were individually suspended from Teflon rods spanning a test chamber filled with oxygen gas at a pressure of 5 p.s.i.a. A 1% aqueous salt fog was periodically superimposed upon the oxygen environment. Finger cots and nylon gloves were used to avoid contaminating these specimens, which were tied to the rods using solvent-extracted nylon lacing tape. A stainless steel tray was placed under the specimens to prevent any beryllium corrosion products from contaminating the chamber.

The chamber was closed, evacuated and then backfilled with pure oxygen to 5 p.s.i.a. A 1% aqueous salt fog was started using a cycle of 6 hours on and 6 hours oif. Pure oxygen was admitted to the chamber to maintain purity within 95 i5 The temperature was maintained at 75i5 F. throughout the test. After 24 hours the test chamber was opened and the specimens were allowed to dry. After washing by immersion in distilled water the specimens were compared with photographs taken prior to the test.

The results are shown in Table 1 below:

TABLE 1 EXAMPLE 2 Fifteen (15) samples of beryllium metal were submitted to an electro-discharge machining treatment in which the samples were exposed to maximum power in order to produce intentional pitting. Within 24 hours after machining the samples began to show signs of corrosion.

The samples were coated with a 0.3% aqueous solution of sodium chloride and permitted to stand for a 24 hour period at room temperature. Severe corrosion resulted.

The 15 samples were then divided into groups of 3 samples each as in Example 1 above and subjected to the same treatment as described for each of the Groups 1 through 5.

All of the samples were exposed to a relative humidity of 95% for five 24-hour cycles.

The results of the humidity test are shown in Table 2:

TABLE 2 Group 6-..- Corrosion and discoloration Group 7.--. Staining and surface discoloration Group 8.-.. No corrosion or surface discoloration Group 9 No visible corrosion-degradation of black paint around very sharp edges of a pitted area Group 10... No visible corrosion-degradation of black paint around very sharp edges of a pitted area It can be seen from the foregoing examples that the process of the instant invention results in a superior method of corrosion protection for beryllium surfaces.

I claim:

1. A process for removing corrosion products from beryllium surfaces and preventing future corrosion thereof which comprises applying to a beryllium surface a corrosion removal solution comprising a dilute oxalic acid solution and a surfactant compatible therewith, said surfactant being present in an amount sufficient to lower the surface tension of said oxalic acid solution, removing said corrosion removal solution from said beryllium surface, applying to the resulting surface a passivation solution comprising major proportions of a hexavalent chromium iron-containing compound and phosphoric acid and a minor proportion of a surfactant compatible therewith, and removing said passivation solution from said surface.

2. A process according to Claim 1 wherein said oxalic acid solution has a concentration of about from 5% to 25%.

3. A process according to Claim 1 wherein said surfactant is a biodegradable linear alkyl aryl sulfonic acid.

4. A process according to Claim 1 wherein said surfactant is present in a proportion of about from 0.01% to 1% by volume in both said corrosion removal and passivation solutions.

5. A process according to Claim 1 wherein said surfactant is present in a proportion of about 0.1% by volume in both said corrosion removal and passivation solutions.

6. A process according to Claim 1 wherein said applying of said corrosion removal solution is conducted at a temperature of about from 100 F. to 160 F. for a period of about from 2 to 4 hours.

7. A process according to Claim 1 wherein said applying of said corrosion removal solution is conducted at a temperature of about from 125 F. to 135 F. for a period of about from 2 to 8 hours.

8. A process according to Claim 1 wherein said hexavalent chromium ion-containing compound is potassium dichromate.

9. A process according to Claim 1 wherein said passivation solution is prepared by mixing equal parts by volume of phosphoric acid and a saturated aqueous solution of potassium dichromate and about from 0.01% to 1% by volume of a biodegradable linear alkyl aryl sulfonic acid and diluting the resulting mixture with up to about 2 parts of water per part of said mixture.

10. A process according to Claim 1 wherein said applying of said passivation solution is conducted at a temperature of about from F. to 170 F. for a period of about from 30 minutes to 1 hour.

11. A process according to Claim 1 wherein said applying of said passivation solution is conducted at a temperature of about from F. to F. for a period of about from 30 minutes to 1 hour.

12. A process for removing corrosion products from electro-discharge machined beryllium surfaces and preventing future corrosion thereof which comprises applying to an electro-discharge machined beryllium surface a corrosion removal solution comprisingan oxalic acid solution having a concentration of about from 5% to 25% and a biodegradable linear alkyl aryl sulfouic acid in a. proportion of about from 0.01% to 1% by volume at a temperature of about from 100 F. to F. for a period of about from 2 to 4 hours, removing said corrosion removal solution from said beryllium surface, applying to the resulting surface a passivation solution prepared by mixing equal parts by volume of 85% phosphoric acid and a saturated aqueous solution of potassium dichromate and about from 0.01% to 1% by volume of a biodegradable linear alkyl aryl sulfonic acid and diluting the resulting mixture with up to about 2 parts of water per part of said mixture, at a temperature of about from 110 F. to F. for a period of about from 30 minutes to 1 hour, and removing said passivation solution from said surface.

13. A beryllium surface which has been treated according to the process of Claim 1.

14. A beryllium surface which has been treated according to the process of Claim 12.

References Cited UNITED STATES PATENTS 3,301,718 1/1967 Morana 148-616 2,030,601 2/ 1936 McDonald 148--6.17 2,164,042 6/1939 Romig 148-624 2,759,862 8/1956 Henricks 148-624 DANIEL J. FRITSCH, Primary Examiner C. WESTON, Assistant Examiner US. Cl. X.R. 

